1. Field of the Invention
The present invention relates to an exhaust gas recirculation (EGR) control apparatus of an engine designed to introduce exhaust gas, or EGR gas, back to a combustion chamber. More particularly, the invention pertains to an EGR control apparatus capable of introducing EGR gas into a combustion chamber even under high-load conditions.
2. Description of the Related Art
Conventionally, EGR systems are widely used to reduce the amount of nitrogen oxides (NOx) contained in exhaust gases by reducing combustion temperature in combustion chambers of an engine. Generally, the EGR system includes an EGR passage connecting an exhaust passage to an intake passage to recirculate part of the exhaust gas from the exhaust passage to the intake passage and into a combustion chamber. The amount of EGR gas recirculated through the EGR passage is regulated by an EGR control valve disposed in the EGR passage according to operating conditions of the engine.
A motive force that forces the EGR gas to flow from exhaust side back to intake side is a difference between exhaust pressure and intake pressure. In the EGR system thus constructed, it is difficult to maintain a sufficient amount of recirculated EGR gas under high-load conditions due to a reduction in the difference between the exhaust and intake pressures. This is because the intake pressure increases due to an increase in the amount of intake air during high-load conditions. A resulting problem is that the conventional EGR system can not exert a sufficient EGR effect, or sufficiently reduce the amount of NOx, under high-load conditions.
A previous approach to the resolution the aforementioned problem is shown in Japanese Unexamined Patent Publication No. 1999-62715, for example, in which an EGR system includes a dedicated compressor disposed in an EGR passage for increasing EGR gas pressure to enable recirculation of the EGR gas even under high-load conditions. Another previous approach is found in Japanese Unexamined Patent Publication No. 2000-329009, in which there is formed a dedicated EGR port besides an intake port and an exhaust port for introducing the EGR gas directly into a cylinder head. An EGR passage of this EGR system is connected to the EGR port, and not to the intake port, so that the EGR gas can be independently introduced into a combustion chamber without any influence of intake pressure in the intake port. The EGR system of Patent Publication No. 2000-329009 further includes an EGR pump disposed in the EGR passage for forcibly introducing the EGR gas into the combustion chamber through the EGR port.
The EGR gas flowing through the EGR passage is a high-temperature gas as it is part of the exhaust gas. The EGR gas may also contain unburned fuel components. If the EGR system is for a diesel engine, the EGR gas may further contain particulates. Pressure control devices, such as the aforementioned dedicated compressor and EGR pump disposed in the EGR passage, are exposed to a fluid having such properties (i.e., a high-temperature gas potentially containing unburned fuel components and particulates), so that these pressure control devices should be of a type having sufficient heat resistance, durability and reliability. Nevertheless, the prior art does not disclose any measures to be taken to achieve this requirement.